1. Field of the Invention
The invention relates to a drum for conveying a sheet, including a drum body, at least one jacket or backing forming a circumferential surface, at least one device for fixing the jacket onto the drum body, grippers for holding the sheet on the circumferential surface, a device for actuating the grippers, journals for mounting the drum in a frame and a drive for rotating the drum about a longitudinal axis.
In printing press, drums or cylinders equipped with grippers are used to convey sheets. The grippers hold the leading edges of the sheets while the sheets rest on the circumferential surface of a drum. A drum is formed of a drum body with a gap of channel formed therein that accommodates a gripper system. The drum bodies of sheet-guiding impression cylinders are produced by casting. The circumferential surface of the drum is produced by machining in the form of chip removal. In general, the circumferential surface is protected against corrosion by a coating.
For reasons inherent to the printing process, the drum or cylinder may be provided with a jacket. The ends of the jacket are held in clamping devices. The jacket is tensioned or tautened about the cylindrical surface of the drum body in the circumferential direction.
German Published, Non-Prosecuted Patent Application DE102 49 731 A1 discloses a clamping and pulling for a cylinder jacket. The jacket is clamped and pulled tight through the use of a wedge transmission.
In a tensioning device, as it is disclosed in German Patent DE 197 41 092C2, insertion rails are provided at the ends of a jacket for a cylinder in a printing process. For tensioning purposes, the insertion rails are fixed in a tensioning rail. The tensioning rail is adjusted in the redial direction through the use of tensioning screws. Thus, the jacket is tautened on the surface of the cylinder.
German Patent DE 43 26 250 C2, corresponding to U.S. Pat. No. 5,503,072, discloses a device for tensioning a printing blanket on a transfer cylinder. In the device, a blocking device acts on one end of the printing blanket to exert pulling forces in the circumferential direction of the transfer cylinder.
The clamping and tensioning devices proposed by the prior art are costly and complex in terms of the required material. When a sheet is guided through a printing nip under pressure, the drum bodies are cohesive and bend-resistant.